Apparatus for producing reentrant magnetic records



July 12, 1949. J H. HAMMOND, JR

APPARATUS FOR PRODUCIHG RBENTRANT MAGNETIC RECORDS 2 Sheets-Sheet 1 Filed April 17, 1946 Jouw HMS H AMMoNb; JR.-

BY oRNEY .H. HAMMOND, JR 2,475,742 APPARATUS FOR PRODUCING v REENTRANT MAGNETIC RECORDS July 12,1949. Y

2 Sheets-SheetY 2 Filed April 17 .m .mm

Patented July 12, 1949 man APPARATUS FOR PRODUCING REENTRANT MAGNETIC RECORDS John Hays Hammond, Jr., Gloucester, Mass. Application April 17, 1946, Serial No. .662,668

7 Claims.

This invention relates to magnetic sound recordings and more particularly to a system for making a reentrant magnetic sound record. on a disc.

An object of the invention is to provide for recording a continuous tone on a reentrant sound track on a disc with accurate phase registration between the joining ends of the 'sound record so that the sound may be reproduced as a continuous tone from the constantly rotating disc.

Another object is to provide a system for synchronizing the speed of rotation of the disc with the signal being recorded so that the sound record will contain an integral number of complete WVeS.

Another object is to provide an automatic synchronizing system of the above type.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

In accordance with one embodiment of the present invention, the disc is driven by a variable speed motor the speed of rotation of which is varied by manual means. A monitoring head is provided for picking up the recorded signal from the disc. The picked-up signal is compared with a signal from the source by a suitable comparing device such as a cathode ray oscilloscope and the speed of the motor is adjusted until the proper wave form appears on the comparing device. rEhe recording is then eifected and the master record thus obtained may be used for making the usual duplicate records therefrom,

The system also provides for deriving from the disc a signal having the frequency of rotation of the disc. The presence of such a signal indicates non-uniformity in the sound record due for eX- ample to a phase shift at the adjoining ends of the record. Synchronization is eiected by regulating the speed of the disc to eliminate this derived frequency.

In another embodiment of the invention the control of the driving motor is eiected automatically by providing one or more alternators operating at frequencies which are harmonics of the frequency of the signal being recorded. The corresponding harmonic is derived from the signal and is applied to an electronic brake which causes the alternator to lock into step with the derived harmonics. The speed of rotation of the disc is thus varied in accordance with the frequency of the signal being recorded so as to attain the correct relationship between the length oi the sound record and the length of each individual wave of which the record is composed.

Although the novel features which are believed to be characteristic of this invention are pointed out more particularly in the claims appended hereto the nature of the invention will be more clearly understood by referring ltothe following (ol. 17a-100.2) Y y.

description taken in connection with the accompanying drawings in which specific embodiments have been set forth for purposes of illustration.

In the drawings:

Fig. l is a block diagram illustrating one embodiment of the invention; and

Fig. 2 is a similar block diagram illustrating another embodiment of the invention in which automatic speed control is eifected.

Referring to Fig. 1 more in detail the invention is shown as applied to the recording of a signal on a magnetic disc l0 which is mounted on a shaft ll driven by a motor lll. A mechanical iilter i3 may be provided to eliminate variations in the drive. The motor l2 may be of the synchronous type normally designed to operate at a iiXed speed from a cycle line I5. For control purposes a source of variable frequency I6 is provided which may be regulated in a range of, for example, from 59 to 61 cycles. The motor l2 is connected by a line l'l to a switch i8 which is adapted to connect the same to either the 60 cycle line i5 or to the control frequency it.

The sound Isource is indicated as a pick up 2li such as a microphone. The pick-up microphone 20 is connected by a line 2l to a signal ampliier 22 and thence through a low pass iilter 23 to va recording head 2li comprising an exciting winding 25 and pole pieces 2G which register with the disc lil and may be oi any standard type for recording magnetically on the disc It the signal received from the pick up device 29. One example of a recording head suitable for this purpose is shown in my co-pendingapplication Serial No. 643,290, filed January 25, 1946, for Sound recording and pick-up device, now abandoned.y

A source 30 of supersonic energy is connected through a high pass lilter 'i and line 'd2 to winding 25 of the recording head Erl. The supersonic energy provides a bias voltage for controlling the magnetic characteristics of the disc i@ and improving the quality of the recorded sound inthe manner well known in the art.

A pick-up head 35, which is similar'to' the recordingA head 2d, is connected by a line 36 to an equalizer 31 which makes the signal correspond to the signal from the pick-up 2@ and thence by a line 38 to one set of sweep electrodes of a comparing device shown as a cathode ray tube it; A signal derived from the amplifier 22 is fed by a line dl to a sweep synchronizer i2 and thence by a line 43 to another pair of sweep electrodes of the cathode ray tube til. The line t3 may, for example, be connected to the horizontal sweep electrodes and the line to the vertical sweep electrodes of the tube All.V

The line 36 is also connected through a switch 45 to a harmonic selecting iter d'5, thence through a line 41 to an amplifier and tuned de- 3 tector indicated at 48, thence by a line 49 to a suitable modulation indicator 5U.

In the operation of this device the size of the record disc l and the speed of rotation thereof is preselected in accordance with the frequency of the signal to be recorded thereon so as to obtain an integral number of recorded waves. I-Iowever, due to variations in the frequency ofthe signal or inaccuracies of the system, exact synf chronism may not be obtained. For a, preliminary check the switch I8 may be actuated to connect the motor l2 to the 60 cycle source |,5 and the tube 40 observed while the signal from the .pick up '20 is being recorded on the disc Synchronism will be indicated by the wave form on the screen of the cathode ray tube. If it is found that this wave form is notl correct, the switch |8 is shifted to connect the motor 2 to the control frequency source I6. 'Ihe frequency of the source I6` is then adjusted until the trace ofthe cathode ray has the correct wave form. When this occurs the recording is correct and. synchronization has been effected to eliminate any phase shift at the joined ends of the sound record.

For a further indication of synchronization, the switch 45 may be closed in which case the filter network 46 is set to select a harmonic of the recorded frequency. This harmonic is amplified and detected in the tuned amplifier detector 48. The detector 48 is tuned to respond to a. signal of the frequency of rotation of the disc |EV and to reject signals of higher frequenc The output of the detector is made available on the indicator 5D which may constitute a head phone or a visual indicator.

With the correct synchronization there will be no signal of the frequency corresponding to the speed of rotation of the disc l0. Hence there will be no indication in the device 50. However, any error in synchronization will cause the selected harmonic to be modulated by a signal of discfrequency and this will appear at the indicator- 50.

The absence of the signal on the indicator 58 will indicate that synchronization has been correctly established not only for the fundamental frequency, but for harmonics up to the selected harmonic.

I n order to obtain a more accurate control the automatic system shown in Fig. 2 may be employed. In this system the record disc 88 is mounted on a shaft 6| and driven through a reduction gear t2V by a motor 63. Mounted on the shaft 64 of the motor 63 are a pair of alternators 65 and 66 which may be set to operate at different harmonics of the recorded signal frequency.

In the system of Fig. 2 the signal pick-up 6l is connected to an amplifier 68 and thence through a. low pass lter t9v and line 1D to a winding 'Il of a recording head 'I2similarto the head 24 described above and positioned to record a signal magnetically upon the disc Si) as the latter rotates.

In this embodiment the supersonic biasing voltage is obtained from the signal amplifierl 68 through a line 13, a filter 14 tuned to pass the fundamental and a harmonic producer, and tuner 15 which is v A plurality of different harmonic frequencies of controlled amplitude. One harmonic derived from thev frequency multiplier 15 which is suitable for a supersonic bias is supplied through a high pass lter 'I6` to thelinel.

Asuitable monitor system 18,includinga, cath- 0de my. tube. Maand-the usuel controls is con:

. may be supplied through nected by a line 19 to the harmonic producer 'l5 and by a line to a pick-up head 8| which is similar to the pick-up head 35 above described. The harmonic producer 'l5 is designed to supply a harmonic or the fundamental to the line '19 according to the comparison desired.

The harmonic producer 'I5 is designed to produce two additional sets of higher harmonics, for example, the fourth and eighth harmonics of the applied signal. The fourth harmonic may be supplied by a line through a phase adjusting network 86 to an electronic brake network 87 which is connected by lines 88 to constitute a load upon the alternator 65. The eighth harmonic line 90 to an electronic brake network 9| which is connected by lines 92 to supply a load across the alternator 66.

The electronic braking networks 81 and 9| may be of any standard type which is designed to cause they alternator to lock into step with the appliedv Voltage. One type is shown for example in this gure wherein the line 9D is connected to the primary of a transformer |00, the secondary of which is connected to the grids |01 of a pair of space discharge tubes |02 operating as a pushpull` amplifier. Thespace discharge tubes include anodes |03 connected in push-pull to the primary |04 of an output transformer, |05 the secondary l Eiof which is connected to lines S2.

The motor 63 is preferably a direct current mctorconnected by lines |87 to a D. C. source and is of the type which is variable in speed in response to changes in torque.

In the; Operation of motorv 63 at approximately the correct speed for making a,- reentrant record. The fourth and eighthharmonics of the applied signal are derivedfrom the frequency multiplier 'i5 and are suppliedv to the electronic brake networks El and 9|- respectively to constitute loads on the alterna-tors |545y and 65, In practice the thermionic brake t-'l` which is responsive to the lower harwithin 1,25*. eliminate any appreciable error in the signal reproducedffrom the'disc 6D. The synchronization can be observed by: the cathode ray pattern as in the case of Fig. 1.

Other-v recordingsv can then be made from the master record without exact synchronization by onthe master disc andwould necessarily be reentrant on the secondary discs.

Byithe abovedescribedsystem I am enabled tornake master recordings of continuous tones on record-discswhichare re-entrant to a high dediscs, for example, are suitable-.for'luse in magnetic musicalinstrument inlmy CQ-pending4 application Serial No. 646,040, iiled February 7, 1946, for Electromagnetic organ.

Although certain specific embodiments of the invention have been shown for purposes of illustration, it is to be understood that various changes and modifications may be made therein as will be readily apparent to a person skilled in the art. The invention is only to be restricted in accordance with the scope of the following claims.

What is claimed is:

1. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, a monitoring pick-up head responsive to the signal recorded on said record, comparing means to compare the signal from said monitoring head with the signal from said pick-up device, and means adjusting the speed of said drive to bring the recorded signal into synchronism with the Signal from said pick-up device so as to obtain a continuous reentrant sound record of the signal.

2. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, a monitoring pick-up head responsive to the signal recorded on said record, a cathode ray oscilloscope having a pair of sweep electrodes connected to said monitoring head and another pair of sweep electrodes connected to be actuated by the signal from said pick-up device for comparing the recorded signal with the picked-up signal, and means adjusting the speed of said drive to bring the recorded signal into synchronism with the signal from said pick-up device so as to obtain a continuous reentrant sound record of the signal.

3. A system for making a reentrant magnetic sound record oi a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, a monitoring pick-up head responsive to the signal recorded on said record, filter means selecting a harmonic of the signal derived from said monitorlng head, means selecting a modulation component of said harmonic having a frequency corresponding to the speed of rotation of said track, means indicating the presence of said component, and means adjusting the speed of said drive to eliminate said component whereby the recorded signal is brought into synchronism with the rotation of the track so as to obtain a continuous reentrant sound record of said signal.

4. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, a pair of alternators driven by said motor and generating frequencies corresponding to different harmonics of the picked-up signal, means deriving the corresponding harmonics from said signal, a load circuit for said alternators and means supplying the respective harmonics to said load circuits to act as brakes on said alternators for causing the latter to lock into step with said harmonics so that the speed of rotation of the track is brought into exact synchronism with the picked-up signal.

5. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, means deriving a pair of different harmonics of the picked-up signal, and means utilizing said harmonies as a variable brake on said motor to cause said motor to operate at a speed such that the circular record track is synchronized with the picked-up signal.

6. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, an alternator driven .by said motor and adapted to generate a frequency which is a harmonic of the picked-up signal, means deriving the corresponding harmonic from said picked-up signal, and means supplying said 'harmonic to said alternator as a load for causing said alternator to lock into step therewith whereby the circular record track is brought into synchronism with the picked-up signal.

'7. A system for making a reentrant magnetic sound record of a continuous tone on a circular sound track of magnetic material, comprising a driving motor connected to drive said circular track at a substantially uniform speed, a sound pick-up device, a recording head adapted to make a magnetic record on said track, a channel connecting said pick-up device to said head, an alternator driven .by said motor and adapted to generate a frequency which isr a harmonic of the picked-up signal, means deriving a corresponding harmonic from said signal, an electronic braking circuit connected to said alternator, means exciting said circuit by said derived harmonic whereby said alternator is caused to lock into step therewith and to bring the rotation of the circular disc into synchronism with the picked-up signal.

JOHN HAYS HAMMOND, Je.

REFERENCES CITED The following references are of record in the nie of this patent:

UNIHTED STATES PATENTS 0 Number Name Date 871,726 Morin Nov. 19, 1907 2,113,226 Young Apr. 5, 1938 

